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 PTH  



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Author12 Posts
  #1

Another question on renal phys please:

I read that one of the mechanisms by which PTH increases concentration of plasma calcium is by decreasing the reabsorption of HCO3.

Can anybody outhere help me understandiong this please? I have problems grasping how and why this ocurs.

Thank you

shaking head





  #2

I know for 3 roles of PTH:

1. in kidney:

a. increased reabsorption of Ca

b. decreased reabsorption of phosphates (why? you know that Ca and phosphates make insoluble Ca-phosphate, so we want to get rid of phoshates in order to have our Ca free)

2. in bones:

increases bone resorption (so increases both Ca and phosphates in plasma)

3. in intestines:

indirect effect: increases absorption of Ca and phosphates (by hormone D3 which is activated by PTH)

Now, you asked about HCO3. I am not sure, but I think this is the reason:

You know that extracellular calcium is found in 3 forms.

1. diffusible (filtrable)

a. ion Ca (free) about 40 %

b. Ca in complex with anions like citrate, phosphate... about 10 %

2. undiffusible (not filtrable)

Ca attached to proteins

Calcium which is attached to proteins serves as a reservoir, so when we need more free calcium in plasma, it can be liberated. How? Recall that proteins and amino acids have both positive and negative groups. Calcium is a positive ion so it binds to negative groups of proteins (COO- group). When you have acidosis (too much H+), H+ as positive ion will also bind to COO- of proteins so there will be less groups to bind with calcium. So calcium will be liberated from proteins and there will be more free calcium in plasma. I want to say that H+ substituted calcium in proteins. I hope that we now understood how increased H+ increases plasma level of calcium. And now, what about bicarbonate ions? When there are less HCO3 (due to decreased reabsorption), what will happen to H+? You know for the formula H+ + HCO3- = H2CO3 (HCO3 buffers H+ ions so decreased HCO3 will increase H+).



  #3

Thank you mildus.Now it makes sens.

So can we generalize that with the same concentration of plasma PTH, increasing of ph will increase plasma calcium concentration then??





  #4

Sorrybut I wanted to write decreasing of ph not incresing of ph.









  #5

Yes! Acidosis - decreased pH (increased H+) will increase plasma calcium concentration. Alkalosis - increased pH (decreased H+) will decrease calcium concentration. That is why alkalosis, by promoting the binding of calcium to albumin, can reduce the fraction of ionized calcium in blood, causing tetany.



  #6

mildus wrote:
I know for 3 roles of PTH:

1. in kidney:

a. increased reabsorption of Ca

b. decreased reabsorption of phosphates (why? you know that Ca and phosphates make insoluble Ca-phosphate, so we want to get rid of phoshates in order to have our Ca free)

2. in bones:

increases bone resorption (so increases both Ca and phosphates in plasma)

3. in intestines:

indirect effect: increases absorption of Ca and phosphates (by hormone D3 which is activated by PTH)

Now, you asked about HCO3. I am not sure, but I think this is the reason:

You know that extracellular calcium is found in 3 forms.

1. diffusible (filtrable)

a. ion Ca (free) about 40 %

b. Ca in complex with anions like citrate, phosphate... about 10 %

2. undiffusible (not filtrable)

Ca attached to proteins

Calcium which is attached to proteins serves as a reservoir, so when we need more free calcium in plasma, it can be liberated. How? Recall that proteins and amino acids have both positive and negative groups. Calcium is a positive ion so it binds to negative groups of proteins (COO- group). When you have acidosis (too much H+), H+ as positive ion will also bind to COO- of proteins so there will be less groups to bind with calcium. So calcium will be liberated from proteins and there will be more free calcium in plasma. I want to say that H+ substituted calcium in proteins. I hope that we now understood how increased H+ increases plasma level of calcium. And now, what about bicarbonate ions? When there are less HCO3 (due to decreased reabsorption), what will happen to H+? You know for the formula H+ + HCO3- = H2CO3 (HCO3 buffers H+ ions so decreased HCO3 will increase H+).


Hi, Mildus, you are correct, but I think Calcium is 40% combined with the plasma proteins, 10% is combined with anionic substances of the plasma and interstitial fluids, and the remaining 50% of the calcium in the plasma is both diffusible through the capillary membrane. and ionized.





  #7

Sorry. You are right.

And do you know the difference in calcium concentration between interstitium and plasma?



  #8

I think is no difference because the plasma and the interstitial fluids have a normal calcium ion concentration of about 1.2mmol/L…..talking about ionized form of calcium…Please complete me or tell me if I am wrong...Ramo<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />







  #9

Ionized calcium concentration is the same.

And what about calcium combined with proteins? You know that plasma proteins concentration is 60 to 80 g/l, and in interstitium, protein concentration is about 20 g/l. So, what about calcium?



  #10

Can someone explain to me the mechanism of tetany in hypocalcemia? I think it has no effect on RMP since membrane is impermeable to Ca at rest. Is there no effect on RMP but only on excitability?

Thanks.



  #11

Interstitial fluid Ca 1.5 mM(.1 % of total) mostly ionized and Plasma 2.5 mM(.5 % of total) half ionized (as u know)



  #12

nod





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